It is commonly required that an electronic signal, either a voltage or current signal, be transmitted from one part of an electronic circuit to another part of the circuit, without any ohmic contact between the two parts. An example of such a case is provided by a circuit of which one part is in contact with a human, such as when a patient is monitored by one part of the circuit, and the other part is connected to external electronic equipment. Providing complete ohmic isolation between the two parts of the circuit ensures that an electronic malfunction in the external electronic equipment will not cause a harmful current to flow through the patient. A widely used electronic circuit for passing signals, while isolating the two parts of the circuit, is a linear isolation amplifier, based on a linear opto-coupler. The term “signal” is used in the present description and in the claims to denote an electronic voltage or current signal, unless otherwise specified.
The linear opto-coupler transmits a signal through one or more pairs of an optical emitter and a receiver, so that the electronic signal is translated first into an optical signal and then back into an electronic signal. Due to the asymmetrical construction of the emitter-receiver pairs and the non-conducting (dielectric) gap transmitting the emitted photons, the signal will travel in one direction only, without any possibility of a reverse current flowing towards the patient. The use of a photonic circuit, however, imposes a condition of unipolarity on the transmitted signal. (For the sake of clarity, the following description refers to positive unipolar signals, as well as positive reference and threshold voltages, but negative unipolar signals, together with negative offset and threshold voltages, may be used in a similar fashion.) In addition to the requirement of positive unipolarity, due to the fact that photonic emitters exhibit linear behavior only above a threshold excitation above zero, the minimum value of the transmitted signal is generally set to be above this threshold.
When the signal to be transmitted is a pure AC-signal, it is converted to a positive unipolar signal before being input to the linear opto-coupler. This conversion may be accomplished by a pre-amplifier within the linear isolation amplifier, which pre-amplifier adds a fixed, positive DC-reference voltage to the AC-signal, thus creating a positive unipolar signal with a sufficiently high positive minimum value. Once the positive unipolar signal has been passed through the linear opto-coupler, its AC-component can be recovered.